Optical disc

ABSTRACT

An optical disc having an aluminum reflective film is disclosed, wherein an oxide of Al in the stable oxidized state is introduced into the Al reflective film to eliminate changes with lapse of time to improve durability of the optical disc. A method for producing a reflective film on an optical disc is also disclosed, wherein a trace amount of oxygen is introduced into atmosphere in the course of formation of the reflective film.

This is a continuation of application Ser. No. 07/834.612, filed on Feb.12, 1992, now abandoned which is a division of application Ser. No.07/667,829, filed on Mar. 12, 1991 now U.S. Pat. No. 5,186,994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical disc, such as a digital audio disc(so-called compact disc) or an optical video disc. More particularly, itrelates to an improved optical disc having an the reflective film.

2. Description of the Prior Art

The optical disc, such as the digital audio disc or the optical videodisc, is repaired by forming a refractive film on the transparentsubstrate, generally formed of polycarbonate or acrylic resin, andforming a hard protective film on the so-formed Al reflective film.

FIG. 1 shows the cross-section of this optical disc. The optical disc 1,shown in FIG. 1, is constituted by a transparent substrate 2, formed ofa synthetic resin, such as polycarbonate or acrylic resin, by reasonmainly of mass-producibility, as mentioned above, a reflective film 3 ofaluminum or the like, formed on the substrate 2 by sputtering orevaporation, and a protective film 4 formed on the reflective film 3 forprotecting the reflective film by spin coating a UV curable resin. Inthe case of a reproduce-only optical disc, such as a digital audio discor an optical video disc, a pattern of projections and recesses,corresponding to the recorded signals, are formed on one surface of thetransparent substrate 1, more correctly, on the surface thereof oppositeto the signal reading surface, that is, on the surface thereof on whichthe reflective film 3 is formed. In the case of a write-once typeoptical disc on which information signals can be recorded, or anerasable optical disc, grooves are formed on the same one surface of thetransparent substrate 1 as that of the reproduce-only optical disc.Although An may be used as the material for the reflective film 3,instead of Al, the latter is usually preferred because it is lessexpensive.

Besides being inexpensive, the Al film is superior in reflectivity andhence used extensively.

Meanwhile, the Al reflective film has a deficiency that the film of Al,being metal, is subject to changes with lapse of time on prolongedstorage due to extraneous factors, and therefore leaves much to bedesired.

For this reason, attempts have been made to form an oxide film on thesurface of the Al reflective film by previous oxidation to provide analuminum-aluminum oxide-aluminum three-layer structure to suppressdeterioration with lapse of time.

However, this three-layer structure of the Al reflective film is alsonot fully satisfactory in durability because, if there be any flaw inthe Al reflective film, corrosion tends to be started thereat to proceedinto the metal aluminum of the mid layer of the three-layer structure.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore a principal object of the present invention to providean Al reflective film less susceptible to changes or deterioration withlapse of time, and an optical disc which, through the use of this Alreflective film, has superior durability.

The present inventors have conducted eager searches towardsaccomplishing the object and, above all, scrutinized into the filmstructure of the Al reflective film to check for the relation thereofwith durability of the disc. As a result, the present inventors havearrived at a conclusion that an Al reflective film is more invulnerableto changes will lapse of time when it is of such a construction that anoxide is dispersed throughout the Al reflective film in its entirety.

The present invention has been fulfilled on the basis of this findingand provides an optical disc comprising a transparent substrate and areflective film on said substrate, said reflective film being composedmainly of aluminum, wherein the reflective film contains an oxide of Aland wherein aluminum and oxygen constituting the oxide of Al bear anoxygen to aluminum atomic ratio of 1.3 or higher.

With the optical disc of the present invention, in which the oxide of Alis dispersed throughout the Al reflective film, the oxidized state ofaluminum oxide is critical. Thus the oxide of Al needs to be a stableoxide. It can be checked from the aluminum to oxygen ratio in the oxideof Al whether or not the oxide of Al is stable. The stable oxide of Alherein means an oxide having the oxygen to aluminum atomic ratio of 1.3or more. Conversely, the oxide is stable if the oxygen to aluminum ratiothereof is lesser than 1.3.

The state of the oxide of Al influences the durability of the Alreflective film significantly. If the oxide of Al is unstable, with theoxygen to aluminum atomic ratio being less than 1.3, it is difficult tosuppress change with lapse of time even if the oxide is dispersed withinthe Al reflective film.

Thus, in accordance with the present invention, the oxide of Al is astable oxide, with the oxygen to aluminum atomic ratio of the oxide ofAl being 1.3 or higher for assuring the durability of the Al reflectivefilm. Although there is no upper limit to the oxygen to aluminum atomicratio, if it is not less than 1.3, there is a limit imposed by thechemical composition of the oxide, such that the ratio Al:O ispreferably 1:1.3 to 1:2.0.

With the Al reflective film of the optical disc of the presentinvention, pure Al not bound with oxygen co-exists with the oxide of Althat is, Al bound with oxygen. Pure Al is necessitated to assure thereflectivity of the reflective film, while the oxide of Al isnecessitated to suppress changes thereof with lapse of time. This needsto be considered in determining the amount of the oxide of Al in the Alreflective film It is practically desirable that the reflectivity of theAl reflective film is 70 percent or higher, and that the amount of theoxide of Al be such that changes with lapse of time of the Al reflectivefilm may be suppressed to as low a value as possible. As the mostpreferred amount of the oxide of Al, pure Al and Al bound with oxygenare contained in the Al reflective film in relative amounts such thatthe amount of Al bound with oxygen accounts for substantially 26 to 33atomic percent.

The above mentioned oxide of Al is produced by introducing a minoramount of oxygen into the atmosphere during formation of the Alreflective film by evaporation or sputtering. The amount of oxygenintroduced into atmosphere may be determined, as a function of the rateof film formation or the like, so that the produced Al reflective filmwill satisfy the above mentioned requirements.

The Al reflective layer may be of an Al monolayer structure into whichthe oxide of Al is dispersed as described above, or may be soconstructed that a thin oxide film is formed on the film surface and alarger amount of the oxide of Al exists near the film surface.

The present invention may be applied to any optical disc provided withThe Al reflective film. Thus it may be applied not only to a digitalaudio disc or a video disc, but to a once-write type disc, such as amagneto-optical disc, or an overwrite type disc, provided that the discis provided with the Al reflective film.

There is also no limitation to the substrate material and any well-knowntype material, including a transparent resin, such as acrylic resin orpolycarbonate, may be employed.

With the oxide of Al in the stable state co-existing in the Alreflective film, it becomes possible to suppress changes with lapse oftime and to improve the durability of the optical disc.

The reflectivity of the Al reflective film may be assured by pure Al(metallic Al) not bound with oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the general construction of anoptical disc.

FIGS. 2 and 3 are diagrams showing a density profile along the filmthickness of an Al reflective film produced by evaporation underintroduction of oxygen, wherein FIG. 2 shows the density profile beforethe accelerated deterioration test and FIG. 3 the-density profile afterthe accelerated deterioration test.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The present invention will be hereinafter explained or the basis ofconcrete test results.

First, Al was evaporated on a polymethyl methacrylate (PMMA) substrate,under introduction of oxygen, for forming an Al reflective film. Thefilm thickness of the produced Al reflective film was of the order of 70to 80 mm.

The produced Al reflective film was analyzed by an X-ray photoelectronspectrometry (XPS). The results are shown in FIG. 2.

With XPS, the kinetic energy of photoelectrons emitted from atoms uponirradiation of X-rays is sequentially measured to obtain an informationconcerning the constituent elements, the quantities and the state ofbonding thereof. Thus the pure Al (metal Al) not bound with oxygen andAl bound with oxygen and thus existing as oxide may be quantitativelydetermined in isolation from each other. It is noted that argon ionsputtering was performed simultaneously for analyzing the Al reflectivefilm along its depth.

FIG. 2 shows the density profile of Al not bound with oxygen, referredto hereinafter as metal Al, Al bound with oxygen, referred tohereinafter as the oxide Al, oxygen and carbon, as measured along thethickness of the Al reflective film. In the drawing, the abscissa standsfor the film thickness in terms of the sputtering time and the ordinatestands for the amounts of the respective elements in terms of atomicpercent.

It is seen from FIG. 2 that the oxide of Al is contained in the Alreflective film evaporated under introduction of oxygen in an amount inthe order of 30 atomic percent as Al. The O to Al atomic ratio O/Al inthe oxide of Al is 1.3 or higher.

The optical disc, on which the Al reflective film has been formed inthis manner, was put to an accelerated deterioration test to check forpossible occurrence of changes with lapse of time. The conditions forthe accelerated deterioration tests are the temperature of 60° C., therelative humidity of 85 percent and the test duration of 200 hours.

As a result, changes in the Al reflective film with lapse of time werescarcely observed even after the accelerated deterioration tests.

FIG. 3 shows the profile of the Al reflective film after the accelerateddeterioration tests. It is seen from this figure that no changes areobserved in the profile of the Al reflective film after the accelerateddeterioration tests.

For comparison, similar tests were conducted on an optical disc on whichthe Al reflective film has been formed without introducing oxygen. Asmall amount of the oxide of Al was contained in the Al reflective filmformed without introducing oxygen, the atomic ratio of O to Al O/Al inthe oxide of Al being about equal to 1. Although substantially nochanges were observed between the density profile before the accelerateddeterioration test and that after the accelerated deterioration test,the Al reflective film after the accelerated deterioration testobviously underwent changes with lapse of time.

From the foregoing it is seen that the present invention provides anoptical disc in which a stable oxide of Al co-exists in the Alreflective film to suppress changes of the Al reflective film with lapseof time to improve the durability of the optical disc.

In addition, the desired reflectivity of the Al reflective film may beassured by aluminum not combined with oxygen to improve the reflectivityof the optical disc.

What is claimed is:
 1. A method for producing a reflective filmessentially composed of aluminum on an optical disc, wherein aluminumoxide is interdispersed within the aluminum, said method comprisingintroducing an amount of oxygen into an atmosphere in the course offorming said reflective film on a transparent substrate of said opticaldisc wherein the amount of oxygen introduced into the atmosphere is suchthat the oxide of aluminum contained in the formed reflective film bearsan oxygen to aluminum atomic ratio between 1.3 and 2.0.
 2. A method offorming a reflective layer of aluminum having aluminum oxide dispersedtherein on an optical disk comprising the steps of sputtering aluminumonto said disk in an atmosphere containing oxygen in an amount such thatsaid aluminum oxide formed in said layer bears an oxygen to aluminumatomic ratio between 1.3 and 2.0.
 3. A method of forming a reflectivelayer of aluminum on an optical disk having aluminum oxide dispersedtherein comprising the steps of evaporating aluminum onto said disk inan atmosphere containing oxygen in an amount such that said aluminumoxide formed in said layer has an oxygen to aluminum atomic ratiobetween 1.3 and 2.0.